This invention relates to an electrode useful in welding. More particularly, it relates to welding a welding nut to a desired metallic substrate.
Projection weld nuts are well known in the art and are generally composed of a metallic element or alloy in the shape of a section of a hollow cylinder, and are thus reminiscent in general shape of a donut, having tiny protrusions or projection points on one of their faces. In general, a weld nut is beneficially applied to reinforce the area around a hole in a piece of sheet metal, which hole in the sheet metal is intended to receive a fastener, such as a bolt. Such reinforcement prevents failure of the sheet metal in the areas adjacent to the fastener when the sheet metal is subjected to physical stress. A weld nut is typically applied to a flat metallic substrate by placing it onto the surface of the substrate at a desired location and connecting one pole of a source of direct electrical current to the metallic substrate. The other pole of the source of direct electrical current is applied to the metallic weld nut, usually under a clamping force and through an electrode which contacts the weld nut. Then a sufficient amount of electrical current is caused to flow through the weld nut and to the flat substrate, which causes the weld nut to be fused or welded to the metallic substrate.
In commercial practice the xe2x80x9cprojection weldingxe2x80x9d of weld nuts (as the procedure is commonly referred to) involves the positioning of the conductive metallic substrate so that the nut will be welded over a guide pin that protrudes from the lower electrode, and then manually or automatically placing the weld nut over the pin and on top of the part. A second electrode is then advanced, typically by a pneumatic cylinder, to apply pressure that squeezes the pieces together. Intensification, a process of increasing the pressure applied after the weld points (electrodes) are closed is often employed. Electric current is then passed through the weld points and through the nut and substrate causing heat to develop between the contact areas of the projections of the steel nut and the steel part. The heat fuses the nut to the part at the projection points of the nut.
A main requirement of the success of the process described above is that the weld nut must be in its proper desired position when the electrical current is applied, for when it is not, such situation may create a finished product that is unsuitable for its intended use by virtue of the weld nut being in an incorrect position. Such mis-alignment may lead to the weld nut being welded out of center with respect to the hole in the substrate, or to only one portion of the circumference of the weld nut being welded to the substrate and partially covering the hole intended to be used to receive a fastener.
A problem solved by using a nut position sensing weld point according the present invention is the elimination of the welding of weld nuts that are not properly placed over the hole in the substrate and are otherwise welded out of location. Through use of the present invention the proper positioning of the weld nut on the substrate prior to the flow of the electrical current is provided, and the possibility of a weld occurring when the weld nut is not in proper position is precluded. Thus, through use of the invention, scrap is greatly minimized.
According to the invention, there is provided an upper electrode which includes a non-conductive portion disposed about the outer circumference of the upper electrode""s lower surface that normally contacts and supplies the weld nut with electrical current. The non-conductive portion extends below the bottom of the upper electrode""s contact surface so as to block contact between the upper electrode and an out-of-position weld nut, thus blocking current flow to mis-aligned weld nuts. A control system used in association with the welding apparatus is capable of responding to the decrease in current flowing in a given process cycle, and may be tailored to alter the process, such as rejecting the part as bad, using electronic means known in the art. The height location of the upper electrode may also be monitored electronically to stop the process even before welding is attempted.
The leading edge of the outer ring of the non-conductive portion may take on any number of physical configurations, such as conical, to funnel a weld nut that is only slightly out of position into its proper desired position so as to assist in increasing the number of successful welds, as well as detect those weld nuts which are too far out to correct. In order to withstand the forces involved with vertical intensification and lateral aligning, it is advantageous that the non-conductive portion be made of a durable, nonconductive conductive material, such as a hard polymer.
In one form of the invention, the non-conductive portion is a hollow cylindrical collar which has a diameter slightly greater than the conductive upper electrode itself and is thus adapted to be interference fit about the outer electrode. In order to prevent the collar from sliding up the upper electrode in such an embodiment, the conductive portion of the upper electrode should be formed with a shoulder the same width as the nonconductive collar so as to give a solid point for the insulator to bear against. The material can also be easily removed to allow for close clearance in special applications. The shoulder that the collar bears against may also be conically shaped to stop outward expansion during intensification on top of a nut. A square shoulder may also be advantageously employed. The collar should be a light press fit so it remains in position but could still be removed for inspection, replacement, or cleaning.
Thus, an upper electrode according to one preferred form of the invention comprises a cylindrically-shaped metallic core portion, the core portion including a base portion having a bottom portion comprising a flat surface, and a tower portion having a top portion. There is a non-conductive outer sleeve portion in the form of a cylindrical shell having an inner diameter, an outer diameter, an upper portion, and a flat lower portion wherein the outer sleeve portion is coextensively disposed about a predominant amount of the base portion along its length dimension and wherein the lower portion of the sleeve portion extends beyond the flat surface portion of the bottom portion of the core portion, and further comprising a bevel annularly disposed about the inner diameter portion of the lower portion of the sleeve portion. The top portion of the tower portion includes a first hole drilled in its center along the centerline of the core portion, and wherein the flat surface portion includes a second hole drilled at its center along the centerline of the core portion, the first hole and the second hole not drilled through to connect with one another.